Logging while drilling tools employing various acoustic measurement techniques are routinely suspended in a borehole and used to measure characteristics of the surrounding formation. Shear slowness, which is the inverse of shear wave velocity, is one such formation characteristic of interest. Examples of acoustic measurement techniques include monopole acoustic logging and multipole acoustic logging. Monopole acoustic logging utilizes a monopole acoustic source to generate, among other things, nondispersive shear head waves that can be measured to determine shear slowness. However, monopole acoustic logging yields meaningful shear slowness measurements only in fast formations in which shear slowness is less than mud slowness (also known as the borehole fluid slowness) and, thus, in which nondispersive shear head waves can propagate.
Multipole acoustic logging utilizes multipole acoustic sources, such as dipole or quadrupole acoustic sources, to generate dispersive waves that can be processed to determine shear slowness. Because it does not rely on propagating shear head waves, multipole acoustic logging can be used to measure characteristics of both fast formations and slow formations. However, multipole acoustic logging, such as quadrupole logging in a logging while drilling (LWD) application, can be relatively insensitive to shear slowness variations in fast formations. Thus, under at least some circumstances, multipole acoustic logging can yield relatively less reliable shear slowness measurements in fast formations than in slow formations.